1. Field of the Invention
The present invention relates generally to the production of curved glass sheets and, more particularly, to an improved method of and apparatus for press bending relatively thin sheets of glass.
2. Description of the Prior Art
Curved sheets of glass are commonly used as glazing closures or windows of vehicles such as automobiles and the like. For such applications, it is imperative that the sheets be bent to precisely defined curvatures determined by the configurations and sizes of the openings as well as the overall styling of the vehicle. Further, it is required that the bent sheets meet stringent optical requirements and that the viewing area of the closures or windows be free of surface defects and optical distortion that would tend to interfere with the clear viewing therethrough. Thus, it can be appreciated that not only is it required to have bending apparatus that will shape glass sheets to precise curvatures, but also that it will do so without causing serious optical defects to the surfaces thereof.
One commercial method of producing such curved sheets generally includes heating pretrimmed, flat sheets of glass to the softening temperature, press bending the heated sheets to a desired curvature between male and female mold members having complementary shaping surfaces and, finally, cooling the curved sheets in a controlled manner to either anneal or temper the glass sheets as dictated by their intended use. Such a bending technique is referred to as "press bending" and may suitably be carried out with the glass sheets oriented vertically, horizontally or obliquely.
In a mass production operation, the above operations are carried out successively while the sheets of glass are being advanced substantially continuously along a fixed path to a heating area, a bending area, and a cooling or tempering area. To achieve satisfactory temper in a glass sheet, the temperature of the glass must be above a predetermined minimum level so as to maintain the core or central portion above a deformation temperature upon being exposed to the tempering medium. The residual heat remaining in glass sheets of conventional thickness such as those having thicknesses ranging from 0.200 to 0.255 inch (5.08 to 6.48 mm), for example, is generally above such predetermined minimum level after bending for immediate advancement to the tempering area and exposure to the tempering medium. Thus, the heat initially imparted to the sheet to bring it to the proper bending temperature can also be utilized in the final heat treating tempering operation.
In recent years, however, in an effort to reduce the overall weight of the automobile as well as reduce production costs, considerable emphasis has been placed on the use of thinner glass sheets for automotive glazing purposes. Tempered side and back windows ranging in the thickness from about 0.125 to 0.156 inch (3.17 to 3.96 mm) are commonplace in today's automobile industry. While the process described above is admirably suited for the mass production of the thicker tempered glass sheets, it does not lend itself to the processing of relatively thinner tempered glass sheets because of the lesser ability of such thinner sheets to retain heat. As the thickness of the glass decreases, the rate of temperature reduction increases. Thus, the heat loss occurring between initial heating and tempering occasioned by the intermediate bending operation in accordance with the above technique brings the temperature of a thin glass sheet down to a level below the aforementioned minimum temperature at which satisfactory temper can be effected. On the other hand, overheating the thin sheets of glass during initial heating to compensate for the rapid subsequent loss of heat during bending renders the sheets extremely pliable, with the attendant loss of deformation control necessary to maintain the shape of the bent sheets within the close tolerances dictated by the automobile design and styling requirements. Moreover, such overheating tends to degrade the surface quality of the finished glass as a result of heat stains, roll deformation, pitting and the like. While attempts have been made to solve these problems in the mass production of thin, bent, tempered glass sheets, only limited success has been achieved in obtaining an acceptable degree of temper while maintaining suitable optical quality and the desired shape imparted to such thin glass sheets during bending.
In past years, the majority of laminated windshields for the automotive industry were bent by the well known gravity, or sag bending technique, wherein a pair of superimposed sheets are simultaneously bent by the forces of gravity on a suitable skeletal-type mold. The technique, although highly successful, is considerably slower and more costly than the press bending process. Moreover, recent advancements in press bending technology have resulted in most instances, in a product that is of much higher quality than that produced by gravity bending. Thus, to provide an improved product and contain costs, there has been a growing trend to bending glass for windshields, when applicable, by the press bending process.
Like the tempered side and back windows, considerable emphasis also has been placed on the use of thinner glass sheets for windshields, as thin as 0.069 in. (1.52 mm), for example. These glass sheets generally are annealed and have much more stringent optical requirements than that required for glass sheets used for the side and back glazing closures of the automobile. As previously mentioned, it is necessary to overheat the glass sheet in the furnace to compensate for the heat that is lost as it is conveyed into the cooler environment associated with the shaping station. Heating the glass sheets to temperatures exceeding the softening temperature of the glass can be especially detrimental to thinner glass sheets on the order of 0.060 to 0.102 in (1.52 to 2.60 mm) or less, for example, rendering them extremely pliable and susceptible to roll distortion. Thus, if not carefully controlled, roll distortion beyond acceptable limits may occur, resulting in a product unfit for sale.
In the typical press bending operation after the sheet is formed between the opposed bending members, the bent sheet is immediately placed on either a roll conveyor or a carrier ring for transport out of the bending station into a cooling station. The lower press member is generally of ring-type construction and in the first method supports the sheet after bending and deposits it on the roll conveyor as the press member is lowered beneath the rolls. The sheet in the latter method is supported by an upper vacuum mold and deposited on the carrier ring immediately after bending. In either instance, during the initial cooling stage the perimeter of the hot glass sheet is in contact with a cooler, substantially continuous ring which accelerates cooling at the edges of the sheet relative to the central portion. This differential cooling has an effect on the ultimate stress pattern established in the sheet after it attains room temperature. When press bending thin glass sheets for windshields, this can result in permanent high stress areas inwardly of the peripheral edge of the sheet which increases the likelihood of breakage resulting from chipping, abrasions, stone hits and the like, during subsequent use in automobiles.
Oftentimes, as dictated by the curvature of the part, it is necessary to heat a portion or portions of the sheet requiring a sharper bend to a higher temperature than the remainder of the sheet to assure satisfactory bending. A windshield with wrap-around pillar areas would be exemplary of such a part. This uneven or differential heating of the sheet further complicates matters since extreme care must be exercised to avoid overheating the entire sheet, which can result in excess crossbend or the formation of objectionable heat stain in the critical viewing area.
A similar situation requiring differential heating occurs when bending glass sheets for the fabrication of an electrically conducting windshield. Such a windshield includes an electrically conducting film used for the defogging and deicing of the vision area. A major surface of one of the sheets is provided with bus bars that provide the necessary circuitry for the subsequently applied transparent electrically conducting coating, as is well known in the art. The bus bars are typically formed of a silver frit material and are applied prior to heating and bending by a suitable process, such as silk screening, for example. Unfortunately, the silver frit has a significant effect on the heating characteristics of the glass, tending to retard the heating rate immediately adjacent the frit lines. The circuitry includes at least one bus bar which extends along one of the lateral edges of the sheet. To compensate for the slower heating rate caused by the presence of the silver frit, it is necessary to heat this portion to a higher temperature than the remainder of the sheet. Thus, it is necessary to establish a differential heating pattern across the width of the furnace to produce a uniform temperature in the sheet suitable for further processing. This has been accomplished with limited success through the regulation of the furnace heating means. However, since the sheet loses heat so rapidly upon exiting the furnace, it is difficult to maintain the sheet at the proper temperature for further processing without overheating within the furnace.
U.S. Pat. Nos. 3,753,673 and 3,854,920, both assigned to Triplex Safety Glass Company Limited, disclose a method and apparatus for controlling heat loss in a glass sheet during the bending operation to minimize "springback" on cooling. "Springback" is a term commonly used to describe the change that occurs in the shape of the glass sheet after bending resulting from the differential cooling of the two major surfaces of the sheet. One or both of the bending dies are heated to a predetermined temperature to balance the overall heat loss between the two surfaces.
One technique successfully developed to produce relatively thin tempered glass sheets is disclosed and claimed in U.S. Pat. No. 4,047,919, assigned to the same assignee as the present invention, whereby open ring-type male and female molds are provided with individually controllable gas burners for reheating at least a portion of the sheet prior to bending. The process addresses the problem of maintaining relatively thin glass sheets, after bending, at a predetermined temperature level suitable for tempering. While it has been successful for its intended purpose, it neither anticipates nor provides the necessary means to overcome the above-described problems associated with bending and tempering very thin glass while maintaining the very stringent optical requirements demanded.